This invention relates to a radio station system, and more particularly to a mobile radio station system having a transmitter and a receiver.
With recent increase in the use of mobile communication systems, for the purpose of simplifying the construction of the mobile radio station system mounted on a vehicle and ensuring the duplex transmission i.e., simultaneous transmission and reception, wave channels are allocated such that the difference between the transmission frequency and the reception frequency of the mobile station is made constant and a plurality of transmission and reception channels are so set as to maintain this relationship.
For this reason, the mobile radio station system is designed to use a single source of multi-frequency signals and when the output of this source is multiplied by a desired number n (being an integer), it is possible to obtain a local oscillation signal of the receiver and a transmission frequency of the transmitter. With this arrangement, it is not necessary to provide a plurality of crystal oscillators for controlling the transmission and reception frequencies. Accordingly, the construction of the mobile radio station system becomes more simple than that of a common transmission and reception system.
FIG. 1 shows the transmission and reception channel allocation in the radio station system of this type. One of channels ch.sub.1, ch.sub.2, ch.sub.3, . . . ch.sub.n is designated by a channel designating input. Letting transmission frequencies associated with the channels ch.sub.1, ch.sub.2, ch.sub.3, . . . ch.sub.n be f.sub.T1, f.sub.T2, f.sub.T3, . . . f.sub.Tn, respectively, and reception frequencies associated with the respective channels be f.sub.R1, f.sub.R2, f.sub.R3, . . . f.sub.Rn, the relation: EQU f.sub.T1 -f.sub.R1 =f.sub.T2 -f.sub.R2 =f.sub.T3 -f.sub.R3 = . . . =f.sub.Tn -f.sub.Rn =f.sub.I
stands. This constant frequency difference f.sub.I stands for the intermddiate frequency to be described later. Of course, the approximate relation EQU f.sub.T1 .about.f.sub.R1 =f.sub.T2 .about.f.sub.R2 = . . . =f.sub.Tn .about.f.sub.Rn =f.sub.I
generally stands.
FIG. 2 is a block diagram showing the construction of a prior art radio station system constructed such that the output frequency f of a source of multi-frequency signal MFG utilizing a synthesizer for determining transmission and reception frequencies (the output frequency f is determined by a channel designating input CS) is multiplied by the desired number n by a multiplier MP.sub.1 located on the receiving side, that the output n.multidot.f of the multiplier MP.sub.1 is applied to a mixer MX as a local oscillation signal for deriving out the difference between it and the reception frequency f.sub.R to produce an intermediate frequency signal f.sub.I which in turn is amplified by an intermediate frequency amplifier IF and then detected and demodulated by a detector DT, and that the output of the detector DT is amplified to a desired power level by a low frequency amplifier AF.sub.1 and then applied to a loudspeaker SP.
The output of the source of the multi-frequency signal MFG is also applied to a modulator MOD to act as a carrier wave which is modulated by the output of a microphone MIC which is amplified by a low frequency amplifier AF.sub.2. The frequency of the modulated wave is multipled by the desired number n by a frequency multiplier MP.sub.2 to produce a transmission signal having a frequency f.sub.T. The transmission signal is amplified by a power amplifier PA and then sent to an antenna ANT through an antenna coupler DP.
A wave dividing circuit or the like may be used as the antenna coupler DP so as to pass to the antenna ANT only the transmission signal and to pass the signal received by the antenna ANT only to a high frequency amplifier RF. After being amplified by the high frequency amplifier RF, the received signal is applied to the mixer MX and then demodulated to form a reception output as described above.
Since the difference between the transmission frequency f.sub.T (being n.multidot.f) and the reception frequency f.sub.R is equal to the intermediate frequency f.sub.I, the following relationships hold. EQU f.sub.I =f.sub.R -f.sub.T ( 1) EQU f.sub.T =n.multidot.f (2)
where f represents the output frequency of the source of the multi-frequency signal MFG. From equations (1) and (2) EQU f.sub.I =f.sub.R -n.multidot.f (3)
As can be noted from equations (2) and (3), since the factor n of multiplication is constant, where the channels of the signals of the transmission frequency f.sub.T and the reception frequency f.sub.R are desired to be changed, the output frequency of the multi-frequency signal source MFG should also be changed. This requires to broaden the frequency band of the circuit, starting from the carrier wave input to the modulator MOD to the output of the modulated wave, in accordance with the channel utilized. At the same time, frequency multipliers MP.sub.1 and MP.sub.2 are required to have broad band characteristics corresponding to the channel used. Since it is impossible to make the high frequency tuning characteristics of the frequency multipliers correspond to a single peak tuning characteristic which is commensurate with only a single frequency, not only the gain is decreased requiring increased number of stages but also the circuit construction becomes complicated which increases the time for adjusting.